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<title>Chapter 6. Summary and Examples</title>
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<h2 class="title"><a id="wrapup"></a>Chapter 6. Summary and Examples</h2>
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<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="wrapup.html#anatomy">Anatomy of a Transactional Application</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="txnexample_java.html">Base API Transaction Example</a>
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</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="txnexample_java.html#txnguideexample">TxnGuide.java</a>
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</dt>
<dt>
<span class="sect2">
<a href="txnexample_java.html#payloaddata">PayloadData.java</a>
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</dt>
<dt>
<span class="sect2">
<a href="txnexample_java.html#dbwriter">DBWriter.java</a>
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</dt>
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</dd>
<dt>
<span class="sect1">
<a href="txnexample_dpl.html">DPL Transaction Example</a>
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<span class="sect2">
<a href="txnexample_dpl.html#txnguideexample_dpl">TxnGuide.java</a>
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<dt>
<span class="sect2">
<a href="txnexample_dpl.html#payloaddataentity">PayloadDataEntity.java</a>
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<span class="sect2">
<a href="txnexample_dpl.html#storewriter">StoreWriter.java</a>
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<p>
Throughout this manual we have presented the concepts and
mechanisms that you need to provide transactional protection for
your application. In this chapter, we summarize these
mechanisms, and we provide a complete example of a multi-threaded
transactional JE application.
</p>
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<h2 class="title" style="clear: both"><a id="anatomy"></a>Anatomy of a Transactional Application</h2>
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<p>
Transactional applications are characterized by performing the
following activities:
</p>
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<ol type="1">
<li>
<p>
Create your environment handle.
</p>
</li>
<li>
<p>
Open your environment, specifying that the transactional subsystem is to be used.
</p>
</li>
<li>
<p>
If you are using the base API, open your database handles, indicating that they are to support transactions.
Otherwise, open your store such that it is configured for transactions.
</p>
</li>
<li>
<p>
Spawn off worker threads. How many of these you need and
how they split their JE workload is entirely up to your
application's requirements. However, any worker threads
that perform write operations will do the following:
</p>
<div class="orderedlist">
<ol type="a">
<li>
<p>
Begin a transaction.
</p>
</li>
<li>
<p>
Perform one or more read and write
operations.
</p>
</li>
<li>
<p>
Commit the transaction if all goes well.
</p>
</li>
<li>
<p>
Abort and retry the operation if a deadlock is
detected.
</p>
</li>
<li>
<p>
Abort the transaction for most other errors.
</p>
</li>
</ol>
</div>
</li>
<li>
<p>
On application shutdown:
</p>
<div class="orderedlist">
<ol type="a">
<li>
<p>
Make sure there are no opened cursors.
</p>
</li>
<li>
<p>
Make sure there are no active transactions. Either
abort or commit all transactions before shutting
down.
</p>
</li>
<li>
<p>
Close your databases.
</p>
</li>
<li>
<p>
Close your environment.
</p>
</li>
</ol>
</div>
</li>
</ol>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Robust JE applications should monitor their worker threads to
make sure they have not died unexpectedly. If a thread does
terminate abnormally, you must shutdown all your worker threads
and then run normal recovery (you will have to reopen your
environment to do this). This is the only way to clear any
resources (such as a lock or a mutex) that the abnormally
exiting worker thread might have been holding at the time that
it died.
</p>
<p>
Failure to perform this recovery can cause your
still-functioning worker threads to eventually block forever
while waiting for a lock that will never be released.
</p>
</div>
<p>
In addition to these activities, which are entirely handled by code
within your application, you also need to periodically back up your
log files. This is required in order to obtain the durability
guarantee made by JE's transaction ACID support. See
<a class="xref" href="jebackuprestore.html" title="Chapter 5. Backing up and Restoring Berkeley DB, Java Edition Applications">Backing up and Restoring Berkeley DB, Java Edition Applications</a>
for more information.
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